Cooler material boosts fuel
cells

By
Kimberly Patch,
Technology Research NewsFinding
a way to reduce the temperature requirements for solid oxide fuel cells,
which now require temperatures of 800 to 1,000 degrees Celsius to use fuel
other than pure hydrogen, is a key step toward widespread adoption of fuel
cells.

Solid oxide is one of five basic types of fuel cells, and is a promising
type for municipal, home and mobile electricity generators.

Researchers from the California Institute of Technology have devised
a cathode that allows solid oxide fuel cells to operate at 500 to 700 degrees
Celsius. This temperature range is high enough to support a variety of fuels,
but low enough that the fuel cell components need not be made from costly
high-temperature materials, said Sossina Haile, an associate professor of
materials science and chemical engineering at the California Institute of
Technology.

The method promises to lower the cost of fuel cells, which could
spur broader adoption of the technology.

Fuel cells extract chemical energy from fuel rather than burning
it like combustion engines. Like batteries, fuel cells contain a pair of
electrodes, and supply a flow of electricity by pushing electrons from the
anode to the cathode. This happens when oxygen reacts with electrons at
the cathode to form oxygen ions, which then migrate through the electrolyte
to the anode where they react with fuel to produce water and release electrons.
Some fuels also produce carbon dioxide at this step.

The researchers' fuel cell looks much like existing cells, said
Haile. "It's like developing a recipe for a cake that bakes at 150 degrees
Fahrenheit rather than 325 degrees Fahrenheit," she said. "The oven still
looks pretty much the same, but it's a lot cheaper because it doesn't have
to withstand such high temperatures."

Until the researchers' new cathode, all known cathode materials
have been effective only at temperatures near 1,000 degrees Celsius at catalyzing,
or speeding up the reaction of the oxygen gas molecule with electrons to
form negatively-charged oxygen ions, said Haile.

Haile's research colleague Zongping Shao developed the material,
made from barium, strontium, cobalt, iron and oxygen, and dubbed BSCF, for
a different application -- oxygen purification membranes.

It was natural that the researchers test the material as a fuel
cell cathode because some of the properties that make the material useful
as an oxygen purification membrane also tend to make it good as a fuel cell
cathode, said Haile.

But the chemical formula was different enough from typical fuel
cell cathodes that the rest of the fuel cell community was not looking in
this direction, said Haile. Fuel cell cathodes traditionally use an alkaline-earth-based
oxide. The researchers' material instead contains an oxide based on rare-earth
metals. Alkaline-earth and rare-earth elements are types of metals than
readily react with oxygen. "It was a classic situation of taking results
from one field and applying them to another that lead to the breakthrough,"
she said.

In addition to catalyzing the oxygen ion reaction at lower temperatures,
the cathode is also particularly efficient at conducting oxygen ions, said
Haile. Conventional solid oxide fuel cell cathodes are made from two different
types of materials -- one that catalyzes the oxygen reduction reaction,
and a second that conducts oxygen ions well to provide an efficient pathway
for oxygen ions to travel to the electrolyte.

The new material does both, said Haile. "It turns out that BSCF
not only has excellent catalytic activity, it also has excellent oxygen
ion conductivity, better than any material we could add as a second component."

"It's hard to believe that a significantly better cathode has been
overlooked because there are so many studies on [similar materials], but
if this is true this cathode material could represent quite a breakthrough
for the low temperature solid-oxide fuel cells," said James Ralph, and assistant
materials scientist at Argonne National Laboratory.

Strontium-cobalt-oxygen materials are good at conducting ions, but
the addition of barium to most similar compositions lowers the conductivity,
so it's not obvious why the combination should work so well, said Ralph.

The new material has the potential to become a widely-used cathode,
but much more research is needed, including long-term performance studies
and independent research to confirm the results, said Ralph.

If the material lives up to its potential, it could also be used
to produce pure oxygen and other useful industrial gases, Ralph added.

The cathode could eventually be used in many kinds of fuel cells,
and because prototype solid oxide fuel cells are already available, it would
not take a great deal of effort to replace the existing cathode with a new
material, said Haile. This could be done within two to three years, she
said.

The new cathode has the potential to speed up practical implementation
of solid oxide fuel cells because the low temperature version is simpler
to make practical, Haile added. "Because reduced temperature operation lowers
the cost of solid oxide fuel cells, we're anticipating that our results
will speed up their commercialization," she said.

From a technological perspective, the next step is to make a complete
fuel cell power generator using the researchers' cathode.

Meanwhile, the researchers are aiming to understand more about what
makes the material a good cathode. Understanding why the material transports
oxygen ions so rapidly and why it catalyzes the reaction well could lead
to even better materials, she said.

The work appeared in the September 9, 2004 issue of Nature.
The research was funded by the Defense Advanced Research Projects Agency
(DARPA).